Vaillant Combi Hot Water Problem

Our new house has a Vaillant VCW-sine T3W Combi.The heating works fine but sometimes when you open a hot tap the boiler fails to fire up. When you switch on the heating then open the tap again you get hot water. I don't know if it is connected but this happened for the first time after I accidently left the hot tap running for about 90 minutes. Could it be the flow switch? The boiler will be replaced in the next 6-12 months so I dont want to spend a fortune on bits that are ok. Any Ideas? Many Thanks